Tear Glands and Drainage
The glands which secrete tears (lacrimal glands) are almond-shaped organs located in the upper part of the socket of the eye. The secreted tears are spread over the surface of the eye by the blinking movement of the eyelids.
There are two small holes on the inner corner of both the eyelids called as lacrimal puncta. Tears get drained out of the eye through these two openings into tubes known as lacrimal canaliculi. The lacrimal canaliculi drain the tears into a sac-like structure on the inner corner of the eye known as the lacrimal sac. The lower part of the lacrimal sac has another tube (called nasolacrimal duct) which drains the tears into the nose.
Types of Tears
Tear secretion is classified into three categories:
- Basal tears: These are secreted continuously by the tear glands and function to moisten, lubricate, nourish and protect the eyes.
- Reflex tears: They are produced by the external stimulus such a dust, smoke, wind, cutting onions or other irritants.
- Emotional tears: These are produced when the person is sad or in pain.
Quantity of tears
The total amount of basal tears produced per day is small, usually ranging from about one ml to three ml per eye.
When tears are spread on the eyeball, the tear film formed is made up of three layers: mucous, water and oil.
- Mucous Layer: This is the layer present directly over the eye and made up of mucous secreted from mucin glands in the eye, which provides lubrication and protection to the eye.
- Water Layer: The middle layer is made up of water and salt, and is produced in the lacrimal glands.
- Oily Layer: The outermost layer is the lipid layer (oil) secreted from Meibomian glands, which are specialised oil glands present on the edge of the eyelids.
Function of Tears
- Delivers nutrients and oxygen and removes waste products from the eye.
- Removes irregularities in the surface of the eyeball and improves the image by making the surface smooth.
- Tears have substances (antibodies, enzymes and other immune components) that defend against microorganisms.
Only humans shed tears for emotional reasons
Vertebrates (animals with backbones) that live on the land produce basal tears and possibly reflex tears. Emotional tears are thought to be seen only in humans. Animals do express grief, distress and cry out, but their cries may not include tears. When they shed tears, it is mostly due to irritation of the eye and no scientific proof is there for emotional tears in animals. However, there are some reports of elephants shedding tears in grief, but this is not confirmed.
Crocodile tears syndrome (Bogorad’s syndrome) is a rare abnormality seen in some people who have an injury to the face region, causing some nerves supplying the salivary glands to get connected to the nerves going to tear glands. When such individuals salivate because of the smell food or while eating, they also shed tears because of this wrong wiring of tear glands with nerve branches of salivary glands. This name is derived from the ancient belief that crocodiles shed tears when they eat a prey (in reality they are lubricating their eyes when they are out of water for a long time).
Dry eye is a condition where the person doesn’t have enough quantity of tears to moisten and nourish the eye. Tear production can reduce due to ageing, various medical conditions and as a side-effect of some medications. Only a small amount of tears is produced that evaporates fast causing the symptoms of dry eye. Read more about Dry Eyes.
In some individuals, the duct draining the eyelids (nasolacrimal duct) gets blocked, and the tears do not get drained from the eye. It leads to an overflow of tears from the eye. Read more about Blocked tear duct.
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- J. Lakshmi Prabha. Tear Secretion-A Short Review. J. Pharm. Sci. & Res. Vol. 6(3), 2014,155-157
- Hitoe S, Tanaka J, Shimoda H. MaquiBright standardized maqui berry extract significantly increases tear fluid production and ameliorates dry eye-related symptoms in a clinical pilot trial. Panminerva Med. 2014 Sep;56(3 Suppl 1):1-6.